Scanning near field ultrasound holography

ABSTRACT

A high spatial resolution phase-sensitive technique employs a scanning near field ultrasound holography (SNFUH) methodology for imaging elastic as well as viscoelastic variations across a sample surface. SNFUH uses a near-field approach to measure time-resolved variations in ultrasonic oscillations at a sample surface. As such, it overcomes the spatial resolution limitations of conventional phase-resolved acoustic microscopy (i.e. holography) by eliminating the need for far-field acoustic lenses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/244,406, filed Oct. 2, 2008, now U.S. Pat. No. 7,798,001, which is acontinuation of U.S. patent application Ser. No. 11/244,747, filed Oct.6, 2005, now U.S. Pat. No. 7,448,269, which is a continuation-in-part ofU.S. patent application Ser. No. 10/913,086, filed Aug. 6, 2004, nowabandoned which claims benefit from and priority to U.S. ProvisionalApplication No. 60/494,532, filed Aug. 12, 2003. The above-identifiedapplications are hereby incorporated herein by reference in theirentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

BACKGROUND OF THE INVENTION

Known acoustic microscopes are used for imaging structures such asintegrated circuit (IC) structures. The spatial resolution, w, of anacoustic microscope is given by:

$w = {0.51\frac{\vartheta}{f.{NA}}}$where θ is the speed of sound in the coupling medium, f is the frequencyof the acoustic/ultrasonic wave, and N.A. is the numerical aperture ofthe lens. For a frequency of 1 GHz, the nominal spatial resolutionattainable is approximately 1.5 μm. Further, the acoustic microscope hastwo other major roadblocks in getting high resolution: (1) impedancemismatches and coupling fluid attenuation that is proportional to f².Higher resolution alternatives for nondestructive mechanical imaginginclude the atomic force microscope (AFM) or scanning probe microscope(SPM) platforms. A few examples include: force modulation microscopy(FMM) as described by P. Maivald, H. J. Butt, S. A. C. Gould, C. B.Prater, B. Drake, J. A. Gurley, V. B. Elings, and P. K. Hansma inNanotechnology 2, 103 (1991); ultrasonic-AFM as described by U. Rabe andW. Arnold in Appl. Phys. Lett. 64, 1423 (1994); and, ultrasonic forcemicroscopy (UFM) as described by O. V. Kolosov, K. Yamanaka in Jpn. J.Appl. Phys. 32, 1095 (1993); by G. S. Shekhawat, O. V. Kolosov, G. A. D.Briggs, E. O. Shaffer, S. Martin and R. Geer in Nanoscale ElasticImaging of Aluminum/Low-k Dielectric Interconnect Structures, presentedat the Material Research Society, Symposium D, April 2000 and publishedin Materials Research Society Symposium Proceedings, Vol. 612 (2001) pp.1; by G. S. Shekhawat, G. A. D. Briggs, O. V. Kolosov, and R. E. Geer inNanoscale elastic imaging and mechanical modulus measurements ofaluminum/low-k dielectric interconnect structures, Proceedings of theInternational Conference on Characterization and Metrology for ULSITechnology, AIP Conference Proceedings. (2001) pp. 449; by G. S.Shekhawat, O. V. Kolosov, G. A. D. Briggs, E. O. Shaffer, S. J. Martin,R. E. Geer in Proceedings of the IEEE International InterconnectTechnology Conference, 96-98, 2000; by K. Yamanaka and H. Ogiao inApplied Physics Letters 64 (2), 1994; by K. Yamanaka, Y. Maruyama, T.Tsuji in Applied Physics Letters 78 (13), 2001; and by K. B. Crozier, G.G. Yaralioglu, F. L. Degertekin, J. D. Adams, S. C. Minne, and C. F.Quate in Applied Physics Letters 76 (14), 2000. Each of these techniquesis traditionally sensitive to the static elastic properties of thesample surface.

Recent developments in atomic force microscopes have involved theapplication of ultrasonic frequency (MHz) vibrations to the sample understudy and non-linearly detecting of the deflection amplitude of the tipat the same high frequencies. With this arrangement, which is commonlyidentified as an ultrasonic force microscope, the ultrasonic frequenciesemployed are much higher than the resonant frequency of the microscopecantilever. The microscope exploits the strongly non-linear dependenceof the atomic force on the distance between the tip and the samplesurface. Due to this non-linearity, when the surface of the sample isexcited by an ultrasonic wave, the contact between the tip and thesurface rectifies the ultrasonic vibration, with the cantilever on whichthe tip is mounted being dynamically rigid to the ultrasonic vibration.The ultrasonic force microscope enables the imaging and mapping of thedynamic surface viscoelastic properties of a sample and hence elasticand adhesion phenomenon as well as local material composition whichotherwise would not be visible using standard techniques at nanoscaleresolution.

The drawback of ultrasonic microscopy is that it measures only theamplitude due to ultrasonically induced cantilever vibrations. Moreover,where the sample is particularly thick and has a very irregular surfaceor high ultrasonic attenuation, only low surface vibration amplitude maybe generated. In such circumstances the amplitude of vibration may bebelow the sensitivity threshold of the microscope in which casemeasurement is impossible. Moreover, none of the above mentionedtechniques measures with high resolution the acoustic phase, which isvery sensitive to subsurface elastic imaging and deep defectsidentification which are lying underneath the surface, without doing anycross sectioning of the samples.

Out-of-plane vibrations created by non-linear tip sample interactionmake a very hard elastic contact with the sample surface. Ultrasonicforce microscopy (UFM) uses the same method except for a amplitudecomponent rather than a phase contrast. If non-linearity is present inthe system, most of the phase contrast will come from the surface andnot from a surface/sub-surface phase contrast. Additionally, non-lineartip sample interaction may not provide results for soft materials.Furthermore, in UFM, high mechanical contrast may be acquired withlittle sub-surface contrast.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a high spatial resolutionphase-sensitive technique, which employs a scanning near fieldultrasonic holography methodology for imaging buried or other subsurfacestructures or variation in the specimen. Scanning near field ultrasoundholography (SNFUH) uses a near-field approach to measure time-resolvedvariations in ultrasonic oscillations at a sample surface. As such, itovercomes the spatial resolution limitations of conventionalphase-resolved acoustic microscopy (i.e. holography) by eliminating theneed for far-field acoustic lenses.

The fundamental static and dynamic nanomechanical imaging modes for theinstrument of the present invention are based on nanoscale viscoelasticsurface and subsurface (e.g., buried nanostructure) imaging usingtwo-frequency ultrasonic holography. The scanning near-field ultrasonictechnique of the present invention vibrates both the cantilevered tipand the sample at ultrasonic/microwave frequencies. The contact,soft-contact and near-contact modes of tip-sample interaction enable theextraction of the surface acoustic waves signal between the twoultrasonic vibrations.

Perturbations to the phase and amplitude of the surface standingacoustic wave may be locally monitored by the SPM acoustic antenna vialock-in and SNFUH electronic module. As the specimen acoustic wave getsperturbed by buried features, the resultant alteration in the surfaceacoustic standing wave, especially its phase, is effectively monitoredby the SPM cantilever. Thus, within the near-field regime (which enjoyssuperb spatial resolution), the acoustic wave (which is non-destructiveand sensitive to mechanical/elastic variation along its path) is fullyanalyzed, point-by-point, by the SPM acoustic antenna in terms of itsphase and amplitude. Thus, as the specimen is scanned across, apictorial representation of specimen acoustic wave's perturbation isrecorded and displayed, to offer quantitative account of the internalfeatures of the specimen.

Certain embodiments provide contact, soft (e.g., intermittent) contact,and/or near contact modes of operation to identify surface andsubsurface (e.g., buried) characteristics of a specimen. Additionally,an SNFUH electronic module extracts surface acoustic phase and amplitudewith or without non-linear tip sample interaction

These and other advantages and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram illustrating the scanning probe microscopewith scanning near field ultrasound holography of the present invention;

FIG. 2 is an illustration of atomic force microscopy of the presentinvention with a vibrating cantilever tip and vibrating sample;

FIG. 3(A) is a schematic illustration of a model nanoparticle system forvalidation of SNFUH.

FIG. 3(B) shows an AFM (topography) image with a featureless top polymersurface.

FIG. 3(C) shows the phase image of SNFUH revealing buried goldnanoparticles with high definition.

FIG. 4(A) shows a schematic of a model test sample for detectingembedded defects/voiding in shallow trenches.

FIG. 4(B) shows an AFM (topography) image with a uniform coating ofdielectric material.

FIG. 4(C) shows a phase image of SNFUH that reveals the surface elasticcontrast and embedded voiding in polymer coating over nitride andhardening of the coating at the trench walls.

FIG. 4(D) shows a line profile across a void marked across X-Y.

FIG. 5(A) shows an AFM topography of malaria-infected red blood cells.

FIG. 5(B) shows an SNFUH phase image from malaria-infected red bloodcells.

FIG. 5(C) represents an AFM topography of early-stage incubation ofparasite infection in malaria-infected red blood cells.

FIG. 5(D) represents an SNFUH phase image of early-stage incubation ofparasite infection in malaria-infected red blood cells.

FIG. 6(A) depicts AFM (topography) imaging of a copper-low K dielectricinterconnect system.

FIG. 6(B) depicts SNFUH imaging of a copper-low K dielectricinterconnect system.

FIG. 6(C) shows a line profile across the voids in FIG. 6(B).

FIG. 7 shows a feedback control circuit used in accordance with anembodiment of the preset invention.

FIG. 8 illustrates the feedback circuit in the context of an embodimentof an electronic readout system used in accordance with an embodiment ofthe present invention.

FIG. 9 illustrates a flow diagram for a method for scanning near fieldacoustic imaging used in accordance with an embodiment of the presentinvention.

The foregoing summary, as well as the following detailed description ofcertain embodiments of the present invention, will be better understoodwhen read in conjunction with the appended drawings. For the purpose ofillustrating the invention, certain embodiments are shown in thedrawings. It should be understood, however, that the present inventionis not limited to the arrangements and instrumentality shown in theattached drawings.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the present invention are directed to anondestructive, high resolution, sub-surface nanomechanical imagingsystem. The system is capable of directly and quantitatively imaging theelastic (static) and viscoelastic (dynamic) response of a variety ofnanoscale materials and device structures with spatial resolution of afew nanometers depending on the ultrasonic frequencies. For viscoelastichigh resolution sub-surface nanomechanical imaging the target maximumprobe frequency is around 5-10 GHz, for example. In an embodiment, themaximum relative phase resolution at this frequency is estimated to be0.001° leading to a viscoelastic time resolution of less than <1 ps. Theinstrument of certain embodiments of the present invention operates in amanner similar to commercially available scanning probe microscopes(SPMs) in that quantitative, digital, rastered, nanometer-scale imagesare obtained of the sample elastic modulus, and sample viscoelasticresponse frequency. The instrument also provides conventional SPMimaging modes including topography, frictional, and force modulationimaging.

Applications for certain embodiments of the present invention arenumerous and represent areas of critical need in Molecular electronics,Nanosystems (NEMS), and Nanotechnology, in general. By combining thenanometer-scale spatial resolution of conventional SPMs with thesub-surface defect identification and imaging capabilities of acousticor ultrasonic microscopes, the instrument fills a critical need incharacterizing and investigating the nanomechanics of nanoscale systems.The SNFUH system and method may be used for: (1) in-vitro imaging ofbiological specimens, tissues and cells, (2) nanomechanical imaging ofburied structures, inclusions in nanocomposites, failure analysis in ICstructures and devices, (3) mechanical properties of low-K materials,(4) stress variation in 3D structures and interconnects, (5) flawimaging in ceramics and quantitative evaluation of mechanicalproperties, etc.

Certain embodiments of the present invention are based on Nanoscaleviscoelastic surface and subsurface (e.g., buried nanostructure) imagingusing two-frequency ultrasonic holography. This is essentially a‘scanning near-field’ ultrasound technique, where both the cantileveredtip 10 and the sample 12 are vibrated at ultrasonic/microwavefrequencies. Contact and soft-contact tip-sample interaction enables theextraction of surface acoustic wave amplitude and phase with highresolution.

In SNFUH mode, perturbation to the standing surface acoustic waveresulting from specimen acoustic wave scattering is monitored by an SPMacoustic antenna. The resulting cantilever deflection merely follows theperturbation to the surface standing acoustic wave, which represents thedissipative lag/lead in the surface response with respect to the tipreference frequency (i.e. the time of flight delay of the specimenacoustic waves reaching the sample surface). Extracting the spatialdependence of this phase term provides image contrast indicative of therelative elastic response of the buried structures, interfaces, andembedded defects to the specimen acoustic wave and thus the resultantperturbation to the standing surface acoustic wave.

Certain embodiments of the present invention provide a system thatmeasures subsurface (e.g., buried) defects, delaminations; cracks;stress migration and etc., while maintaining the high resolution of theatomic force microscope. It utilizes (1) an atomic force microscopysystem having a cantilever 14 with a tip 10 at a free end sitting on topof the vibrating device 16 for supplying vibrations to the cantilever ata frequency greater than cantilever resonance frequency, (2) a sample 12having a vibration device 18 sitting under it for providing highfrequency excitations and (3) an optical detector or other detector fordetecting movement of the cantilever. It detects the beats, products,additions frequencies and beats, products of their harmonics andmodulated waveforms, when the vibrating tip interacts with the vibratingsample, which falls within its detection range. With this embodiment, itis possible to recover surface acoustic wave phase information of thetip-surface mechanical interaction, which allows measurement ofviscoelastic properties and enables the application of acousticholography algorithms for imaging nanoscale sized sub-surface (e.g.,buried) defects. The microscopy apparatus utilizes scanning Near FieldUltrasound Holography (e.g., SNFUH) for high resolution nanomechanicalimaging of buried defects and structures.

The surface acoustic wave's amplitude and phase are experimentallyextracted from the tip deflection signal via lock-in detection. Thephase sensitivity of this measurement is involved in extractingtime-resolved mechanical properties of materials as well as potentiallyenabling subsurface imaging (e.g., buried nanostructures).

Certain embodiments of the present invention detect the phase oftransmitted acoustic wave directly at wafer/device surface. Further,certain embodiments of the present invention detect the phase of surfaceacoustic wave directly at wafer/device surface. Further, certainembodiments of the present invention utilize scanning nanoprobe phasedetection so as to eliminate the need for acoustic lenses. The NanoprobeAcoustic Antenna (AFM Tip) of certain embodiments of the presentinvention is advantageous because, for example, it provides induction ofMHz-GHz nanoprobe mechanical oscillations via high frequency flexuralmode excitation, i.e. the mechanical wave guide and the cantilevermonitors the phase shifts between tip 10 and sample 12acoustic/ultrasonic vibrations.

As shown in FIGS. 1 and 2, two oscillations are applied to the tip 10and sample 12 by two matched piezo crystals 16 and 18 attached to the Sisubstrate of the tip and the base of the sample, respectively. Eachpiezo 16, 18 is driven by a separate waveform with a SNFUH electronicmodule 36 providing the input frequency to an RF lockin amplifier 40 forsurface acoustic wave (SAW) amplitude and phase extraction. The SNFUHelectronic module 36 selects beats, product, and/or addition frequenciesfor example, to aid in performing holography in contact, soft-contact,and near-contact modes. Additionally, the SNFUH electronic module 36allows SNFUH to be performed in the linear regime of tip-sampleinteraction. In an embodiment, the SNFUH electronic module 36 includes amixer circuit, variable resistor(s), op-amp(s), band pass filter(s),and/or other filters for mixing frequency signals and selectingfrequency products, additions and beats, for example.

Any Scanning Probe Microscope (SPM) may serve as the base platform. Asignal access module (SAM) 22 is used as the input site for SNFUH, andmodulus-calibration signals. The integrated piezo (for high frequencyexcitation) will enable ultrasonic excitation of higher-order flexuralresonances of the cantilever tip 10 to provide the ultrasonic vibration.

The sample ultrasonic vibration is driven by function generator 32.Second function generator 34 applies the sample ultrasonic vibration.The resulting differential output signal from detector is accessed withthe signal access module (SAM) 22 and acts as the input to RF lockinamplifier 30 or similar lockin amplifier for extraction of SAW amplitudeand phase. The lockin response signal constitutes a SAW amplitude andphase which act as a input into the signal acquisition electronics 46,via the SAM 22, for image display and analysis. The SNFUH electronicmodule circuit 36 extracts beats, products of fundamental and harmonics,and/or modulated waveform(s), which serve as a reference for a RF lockinamplifier 40 or other lockin amplifier, for example. The differentialoutput of optical detector (A-B) is input, via the SAM 22, into the RFlockin 40. The resulting output constitutes the SNFUH image signal. Acomputer 44 or other processor running data acquisition/analysissoftware, such as Lab View or other data acquisition and/or analysissoftware, acquires both the A-B signal from the digital scope and thelockin. In an embodiment, a switch may be included to select an SNFUH orUFM signal for acquisition, for example.

In an embodiment, the sample piezo consists of aninsulator/electrode/piezo/electrode/insulator blanket multilayer (e.g.,10 cm×10 cm) stack. The insulators consist of epoxied machinableceramics or thin, spin-cast polymer coatings, dependent upon ultrasoniccoupling efficiency. The Cr/Au electrodes or other similar electrodesprovide electrical contact between the piezo and the second functiongenerator 34. The assembly is counter-sunk into a modified SPM samplemount.

As shown in FIG. 2, using SNFUH, a high frequency acoustic wave islaunched from below the specimen 12, while another high frequencyacoustic wave is launched at least a slightly different frequency islaunched on the SPM cantilever 10. The SNFUH electronic module 36 isused to spatially monitor the phase perturbation to a standing surfaceacoustic wave, which results from a scattered specimen acoustic wave.The resonant frequency of the cantilever, f₀, may be in the 10-100 kHzrange, for example.

Certain embodiments may also include a feedback circuit, such as thecantilever resonance feedback circuit 50 depicted in FIG. 7. Thefeedback circuit 50 includes a first op amp (OA) 52, a second op amp(OA) 54, a phase compensator (PC) 56, a voltage-controlled oscillator(VCO) 62, a waveform or function generator 68 and a cantilever tiphaving a piezo transducer 74 interacting with a sample 78.

For SNFUH operation to be uniquely calibrated across samples, phase ofthe cantilever may be fixed. In order to fix the phase of the tip, aresonance feedback circuit, said as feedback circuit 50 may be employed.The feedback circuit 50 maintains the tip carrier frequency at resonanceand fixes or sets the phase, so that the tip phase is a stable referencefor sample phase. For example, at higher frequencies (e.g., 150 MHz-10GHz), the cantilever easily strays from its resonance, and feedback maybe used to maintain the cantilevered tip frequency at resonance. Boththe sample and cantilever may maintain their resonance frequencies togenerate a high resolution viscoelastic response. In an embodiment, thesystem may operate in feedback mode with the feedback circuit 50generating feedback for frequency resonance, or the system may operatewithout the feedback circuit 50 activated.

In the feedback circuit 50, the voltage-controlled oscillator 62 drivesthe tip piezo transducer 74. The VCO 62 is connected through the phasecompensator 56, which acts as an input to an op amp pair 52, 54 forfeedback control. If the cantilever resonance frequency shifts duringscanning, the reduction in tip vibration amplitude will reduce thevoltage across the piezo transducer on the cantilever. This voltage willcause a shift in the PC 56 output. The shift in PC output will bring theVCO 62 back into resonance.

FIG. 8 illustrates the feedback circuit 50 in the context of anembodiment of an electronic readout system 800 used in accordance withan embodiment of the present invention. The electronic readout system800 may be a MOSFET embedded electronic readout, for example. Usingembedded MOSFET as electronics feedback may provide a currentsensitivity of ΔI_(d)/I_(d)=10⁻⁶/nm of cantilever bending. Deflectionsensitivity of the electronic readout may be of the same order asoptical feedback detection, for example. In an embodiment, deflectionsensitivity may be approximately three orders of magnitude higher thanexisting passive and active detection technologies, such aspiezoresistive detection. In an embodiment, a high signal-to-noise ratioand minimal 1/f noise allow the MOSFET embedded electronic readout to beused for electronic feedback in SPM's (scanning probe microscopes), forexample.

The feedback circuit 50 may be used to control a power supply 84 whichsupplies power to a piezo 86. The piezo 86 includes contacts, such as Au(gold) contacts 88, as well as an actuator 90 and a BiMOS transistor 92.The piezo 86 is driven by an oscillator 94. Feedback from theoscillating piezo 86 is gathered by the electronic detection unit 96.The feedback signal from the electronic detection unit 96 is convertedusing the analog to digital converter (ADC) 98 and fed into the feedbackcircuit 50 for control of the power supply 84. Set point 100 provides abasis or reference for operation of the feedback circuit 50. Feedbackfrom the circuit 50 helps to ensure that the tip and the sample arebeing vibrated at their respective resonance frequencies, for example.

An example of viscoelastic nanomechanical imaging is shown in FIG. 3.FIG. 3(A) shows gold nanoparticles dispersed on a polymer coatedsubstrate are buried under an approximately 500 nm thick polymer layer.Use of a model polymer-nanoparticle composite demonstrates the highlateral spatial resolution and depth sensitivity of the SNFUH approach.A specimen consisting of gold nanoparticles buried deep underneath apolymer cover layer was prepared by dispersing colloidal goldnanoparticles on a polymer (poly(2-vinylpyridine)—PVP) coated siliconsubstrate. The gold nanoparticles have an average diameter of 15 nm andare well dispersed on the film surface. The nanoparticles were thenfully covered with another polymer film about 500 nm thick, as shownschematically in FIG. 3(A). The normal AFM topography scan, FIG. 3(B),shows a smooth featureless surface of top polymeric layer with surfaceroughness of approximately 0.5 nm. On the other hand, the phase image ofSNFUH, FIG. 3(C), shows well-dispersed gold nanoparticles buriedapproximately 500 nm deep from the top surface. The contrast in thephase image of SNFUH arises from the elastic modulus difference betweenthe polymer and gold nanoparticle, which induces the time dependentphase delay of the acoustic waves reaching the sample surface.

To demonstrate the efficacy of SNFUH in identifying underlying defectsin narrower trenches, shallow trench structures may be fabricated asshown in FIG. 4(A). The trenches are etched in SOD (spin-on-dielectric)with a 50 nm thin layer of LPCVD Si₃N₄ as a capping layer and then Si₃N₄is etched down in the 1 μm deep trenches using the wet processing.Trench width in the example is about 400 nm. A 500 nm thick layer ofpolymer [Benzocyclobutene (BCB)] was spin-coated followed by thermalannealing for curing the polymer.

FIG. 4(A) shows a schematic of series of isolated shallow trenchstructures. FIG. 4(B) shows a conventional AFM topography image, whileFIG. 4(C) is a corresponding (simultaneously recorded) SNFUH phaseimage. The typical 7.5×7.5 μm² topography scan shows uniform andcontiguous polymeric coating on SiN and inside the trenches. On theother hand, the corresponding SNFUH phase image shown in FIG. 4(B)reveals phase contrast reminiscent of embedded voiding within thepolymer, and at the SiN-polymer interfaces. The dark contrast in thephase image in polymer coated SiN lines corresponds to voids atpolymer-SiN interface, i.e. voiding underneath the contact. The contrastis due to the distinct viscoelastic response from the specimen acousticwave from the voids, for example. A hardening of the polymer in thetrench and its sidewall is also evident in the phase image, whichresults from thermal annealing and possibly poor adhesion with SOD, forexample. FIG. 4(D) shows a line profile of phase across X-Y from FIG.4(C). A subsurface phase resolution of 50 mdeg may be achieved, forexample. Current methods of diagnosis employ destructive approaches suchas wet etching followed by SEM imaging, which are undesirable. Thus,SNFUH may be an improved tool-set for such subsurface metrology needs.

The efficacy of SNFUH to imaging embedded or buried sub-structures inbiology is demonstrated in FIG. 5, which depicts high resolution andremarkably high contrast arising from malaria parasites inside infectedred blood cells (RBCs). FIG. 5 demonstrates early stage direct andreal-space in-vitro imaging of the presence of parasites inside RBCswithout labels or sectioning of cells, and under physiologically viableconditions. Plasmodium falciparum strain 3D7 was cultured in-vitro by amodification of the method of Haldar et al. Parasites were synchronizedto within 4 hours using a combination of Percoll purification andsorbitol treatments, cultured to 10% parasitemia, and harvested at theindicated times, for example.

SNFUH imaging may be performed using the near-contact mode method forimaging soft structures, for example. An SNFUH electronic module may beused to bring the cantilever in near-contact mode and then the samplewas subsequently scanned over the RBCs while maintaining the near-fieldregime. FIGS. 5(A) and 5(B) show AFM topography images and SNFUH phaseimages from infected RBCs, respectively. The AFM topography image showsthe typical surface morphology of infected RBC, while the SNFUH phaseimage shows high contrast from the parasite residing well inside theRBC. In addition to several other features reminiscent of membraneproteins and sub-cellular contents, multiple parasites are clearlyevident. In order to further demonstrate the capability of SNFUH forearly stage diagnosis of parasite infection, RBCs incubated for onlyfour hours are examined, which is difficult to validate by othernon-invasive technique (e.g., fluorescence tagging). FIGS. 5(C) and 5(D)show a pair of images similar to those in FIGS. 5(A) and 5(B). SNFUH maybe sensitive to early stage parasite infection in RBC, as reflected byimage contrast consistent with parasite infection, for example.

FIG. 6 shows a series of low-K dielectric polymer and copper lines withlateral dimension of about 200 nm for the polymer and around 60 nm forcopper. FIG. 6(A) shows the conventional topography image, while FIG.6(B) is the corresponding (simultaneously recorded) SNFUH phase image.The typical 1400×1400 nm² topography scan shows uniform and contiguouspolymer and copper lines. However, the corresponding SNFUH phase imageshown in FIG. 6(B) reveals phase contrast reminiscent of sub-surfacevoiding in copper lines. FIG. 6(C) shows a line profile across thevoids. The dark contrast in the phase image of copper lines correspondsto voids underneath the metal. The presence of this contrast in phaseimage implies that there is insufficient metal filling at the bottom,i.e. voiding underneath the contact, which undergoes a distinctviscoelastic response. Interestingly, a hardening of the polymericregions and its sidewall is also evident in the phase image, whichresults from RIE processing and chemical-mechanical-polishing (CMP).SNFUH may serve as a tool-set for such sub-surface metrology challenges.

Thus, SNFUH may be used to facilitate: (1) quantitative high resolutionnanomechanical mapping of subsurface (e.g., buried) structures toidentify process-induced mechanical variations and/or nanoscale cohesivedefects; (2) nanomechanical viscoelastic (dynamic) imaging tospecifically investigate surface and subsurface interfacial adhesive(bonding) response, etc.

Other applications for the system and method of the present inventioninclude: (1) non-destructive imaging of subsurface defects in 3Dinterconnects and stress migration along the devices due to electricalbiasing; (2) non-destructive inspection for interconnect nanotechnologyfor nanometer-scale resolution, to enable imaging of electro-mechanicaldefects (e.g. nanotube contacts) and to enable imaging of nanoscaleintegrity in molecular interconnect assemblies; (3) subsurfacenano-cracks, stress, delamination identification in ferroelectrics,ceramics and micromechanical structures and devices; (4) non-destructivedefect review and process control in integrated IC materials and devicesto provide modulus measurement for soft materials (i.e. porousdielectrics) and to provide void and delamination defect detection toavoid off-line, cross-sectional failure analysis; (5) self assembledmonolayers and subsurface defects in biological cells and materials,in-vitro imaging of biological cells, tissues and membranes, nano-biomechanics and (6) quantitative extraction of elastic modulus with highaccuracy.

FIG. 9 illustrates a flow diagram for a method 900 for scanning nearfield holography imaging used in accordance with an embodiment of thepresent invention. First, at step 910, a sample is positioned withrespect to a cantilever for nanomechanical imaging of the sample.Surface and/or sub-surface imaging may be performed with respect to thesample, such as a tissue or other sample. At step 920, a tip of thecantilever is vibrated at a first frequency. The frequency may be afirst microwave, ultrasonic, or other acoustic frequency, for example.Then, at step 930, the sample is vibrated at a second frequency, such asa second microwave, ultrasonic, or acoustic frequency, for example. Inan embodiment, the second frequency vibrates the sample at a frequencythat is offset from the first frequency vibrating the tip. In anembodiment, the tip and sample piezos are vibrated at their respectiveresonance frequencies.

Next, at step 940, interaction between the vibrating tip and thevibrating sample is detected. The interaction may be a physicalinteraction and/or a non-contact signal interaction between the tip andsample, for example. The interaction may constitute movement of the tip,for example. Tip movement may be provided as a tip deflection signal,for example. In an embodiment, the interaction may include a linearand/or non-linear interaction between the tip and the sample. In anembodiment, SNFUH may be performed using the linear tip-sampleinteraction in soft contact and near contact mode to obtain the highresolution sub-surface phase. SNFUH may be performed in soft and nearcontact modes to obtain the sub-surface information, such as burieddefects or variations.

At step 950, amplitude and phase information associated with the surfaceacoustic waves of the sample are extracted. Amplitude and phaseinformation may be extracted from the tip deflection signal using lockin detection, for example. In an embodiment, subsurface mechanical data,such as interfacial bonding, regarding the sample may also be extractedfrom the tip deflection signal.

At step 960, surface and/or subsurface characteristics of the sample maybe imaged using the amplitude and phase information. In an embodiment, aspatial variation of surface and subsurface viscoelastic phase may beimaged, for example. In an embodiment, a characteristic viscoelasticresponse time of the sample may be defined based on the amplitude andphase information. Then, at step 970, vibration of the cantilever tip ismaintained at a tip piezo resonance frequency, and vibration of thesample is maintained at a sample resonance frequency. Feedback, such aselectrical feedback, may be provided to maintain the tip resonancefrequency and the sample resonance frequency.

In an embodiment, product frequencies may be used with optical detectionto obtain biological imaging with high subsurface resolution. The sampleand cantilever are excited at their fundamental resonance frequencies(e.g., 1.96 MHz and 3.28 MHz, respectively). Additionally, theindividual sample and cantilever carriers are modulated with one or moremodulation frequencies (e.g., 25 kHz and 35 kHz, respectively). Next, acombination of a SNFUH electronic module and RF lock in band pass filteroutputs a product of the two modulated waveforms. The product output isthen fed into a RF lock in amplifier reference input.

Using product frequencies allows improved selection of carrierfrequencies. In an embodiment, the larger the frequency of acousticoscillations, the higher the order of phase contrast that may beobtained from SNFUH images. Thus, smaller features not seen at lowercarrier frequencies may be detected using a higher frequency carrier.Additionally, use of product frequencies allows use of non-matching tipand cantilever piezos.

In an embodiment, forces between the cantilever and sample may becontrolled during SNFUH operation in near contact mode. Contacting thecantilever with the biological samples may rupture the samples. However,near contact operation allows monitoring and subsurface imaging of softstructures. Near contact mode operation may provide sub-surface imagingof soft structures as well as providing quantitative analysis ofbiological structures, cells and/or tissues, for example.

In an embodiment, beat frequencies may be used to monitor samples innear contact mode. Alternatively, frequency addition may be used forsample monitoring in near contact mode. In an embodiment, harmonics, aswell as or in addition to fundamental frequencies, may be used in beatfrequency, product frequency, and/or frequency addition (sum) operation.For example, the system may perform optical and/or electronic detectionaccording to a variety of frequency strategies up to 1000 MHz with avery thin film of ZnO.

In an embodiment, cantilever and sample carrier frequencies may bemodulated with amplitude modulation. For example, two carriers, acarrier for the cantilever and a carrier for the sample are amplitudemodulated individually. In this configuration, the tip-sample assemblymay be excited with a higher frequency (with or without matchingpiezos). Then, an amplitude modulated waveform may be obtained from boththe cantilever and sample and input to a SNFUH electronic module. Theoutput of the electronic module is a product/difference/additionfrequency. A beat or difference frequency is a difference betweenmodulation frequencies, for example.

In an embodiment, an electronic readout device may be implemented withthe sample monitoring system. An example of such a readout device may bethe readout device described in U.S. patent application Ser. No.10/996,274, filed on Nov. 23, 2004, entitled “Method and System forElectronic Detection of Mechanical Perturbations Using BiMOS Readouts”,which is herein incorporated by reference. In an embodiment, using areadout circuit allows product frequencies to be used without modulationsince operation is not limited by response time as with opticalphotodiodes, for example.

In an embodiment, use of electronic detection eliminates opticaldetection of amplitude and phase removes or eases limitations imposed bya photo-detector response frequency, such as a 1 MHz photo-detectorresponse frequency. Electronic detection aids in fabricatingmulti-active probes with on-chip integrated piezo-actuator (e.g., ZnO)and embedded MOSFET feedback electronics. Additionally, electronicdetection does not limit detection of subsurface features based on beatfrequency. Multiples of frequencies may be used to enhance bothamplitude and subsurface phase contrast, and thus the viscoelasticresponse. Enhanced viscoelastic response results in enhanced phasecontrast from features less than 50 nm, for example, which may bedifficult to detect using only beat frequencies.

Thus, certain embodiments provide a scanning near field ultrasoundholography (SNFUH) technique to image high resolution buriednanostructures, defects, 3D tomography, identification of individuallayers in multilayer thin film stacks and dopant mapping, for example.Certain embodiments integrate three approaches: a combination ofscanning probe microscope platform (which enjoys excellent lateral andvertical resolution) coupled to micro-scale ultrasound source anddetection (which facilitates “looking” deeper into structures,section-by-section) and a holography approach (to enhance phaseresolution and phase coupling in imaging). Certain embodiments providenear field, ultrasonic holography, near field microwave holography, orother near field acoustic holography for surface and subsurface imagingin nano- and micro-specimens, such as biological, mechanical, andelectrical specimens. Certain embodiments allow SNFUH imaging usinglinear and/or non-linear interactions between cantilever and specimen incontact, soft contact and/or near contact mode, for example.

As a result, the technique allows subsurface flaw imaging in nano- andmicro-composites, MEMS, CMOS, and heterostructures, for example. Thetechnique also provides in-vitro imaging of biopolymer, biomaterials andbiological structures (e.g. viewing cell-membrane or implant-biointerface). Additionally, certain embodiments detect voiding andsubsurface defects in low-K dielectric materials and interconnects, aswell as stress migration and defect analysis in 3D interconnects andMEMS. Certain embodiments facilitate dopant profiling and modulusmapping in non-contact mode and also provide non-invasive monitoring ofmolecular markers/tags-signal pathways, for example.

In an embodiment, a high frequency (e.g., on the order of hundreds ofMHz) acoustic wave is launched from the bottom of the specimen, whileanother wave is launched on the AFM cantilever. These acoustic waves aremixed together through a SNFUH electronic module, which includes acombination of various filters, mixers, feedback electronics andelectronic components used to obtain a desired product and addition offundamental resonances and related harmonics (in addition to differencefrequencies). The resulting mixed wave is monitored by the AFM tip,which itself acts as an antenna for both phase and amplitude. As thespecimen acoustic wave gets perturbed by buried features, especially itsphase, the local surface acoustic waves are very effectively monitoredby the AFM tip. Thus, within the near-field regime (which enjoys superblateral and vertical resolution), the acoustic wave (which isnon-destructive and sensitive to mechanical/elastic variation in its“path”) is fully analyzed, point-by-point, by the AFM acoustic antennain terms of phase and amplitude. Thus, as the specimen is scannedacross, a pictorial representation of acoustic wave's perturbation isfully recorded and displayed, to offer a “quantitative” account ofinternal microstructure of the specimen.

The SNFUH system is operational in the linear and near-contact regime oftip-sample interaction and proves effective for in-vitro imaging ofbiological cells and tissues using the SNFUH electronic module, forexample.

Thus, certain embodiments provide an electronic readout based on anembedded MOSFET to detect product frequencies, which is thereby notlimited by an optical detector. Moreover, electronic readout may help inbuilding a parallel SNFUH system for industrial application. Inaddition, a Brillion Zone Scattering technique may be used to map themodules of any surface in non-destructive way with greater efficiencythan other methods.

Certain embodiments may be applied to microelectronics, especially as anadvanced nanoscale surface and sub-surface metrology tool-set. Further,certain embodiments provide imaging for Nanoelectronics, reliability andfailure analysis in Microsystems (MEMS), and Nanotechnology, in general,and especially biomolecular interconnects and BioMEMS. Additionally,certain embodiments provide in-vitro imaging of biological structureswithout having to “open-up” internal structures. By combining thenanometer-scale spatial resolution of conventional SPMs with thesub-surface imaging capabilities, certain embodiments may characterizethe surface defects and structures with high resolution and will havefurther potential for developing nanoscale non-invasive 3D tomography,for example.

Scanning Near Field Ultrasound Holography (SNFUH) may be used, forexample, in near contact and contact mode with product frequencies forthe following structures and devices: (1) Investigating mechanicaluniformity and process-induced mechanical modification of materials inintegrated circuit (IC) structures and MEMS; (2) Real-time in-vitrobiological imaging of red blood cells infected with malaria parasites;(3) Voiding in copper interconnects and (4) Non-invasive monitoring ofnanoparticles buried under polymeric films. Such capabilities maycomplement cross-sectional imaging techniques such as SEM-EDS (scanningelectron microscope-energy dispersive spectroscopy), TEM-EDS(transmission electron microscope-energy dispersive spectroscopy),TEM-EELS (transmission electron microscope-electron energy-lossmicroscopy), and ex situ STM (scanning tunneling microscopy) toinvestigate the nanomechanics and subsurface imaging of materialinterfaces, the uniformity of conformally deposited coatings, andmechanical defects in multilayer structures, for example.

Many other applications of the present invention as well asmodifications and variations are possible in light of the aboveteachings. While the invention has been described with reference tocertain embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from its scope.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed, but that the invention will include allembodiments falling within the scope of the appended claims.

1. A scanning near field ultrasound holography method for surface andsubsurface imaging, comprising: vibrating a cantilevered tip at a firstultrasonic frequency; vibrating a sample at a second ultrasonicfrequency, the first ultrasonic frequency being different from thesecond ultrasonic frequency; detecting movement of the cantilevered tipinteracting with the vibrating sample to provide a tip deflectionsignal; and extracting surface acoustic wave amplitude and phaseinformation associated with the surface and subsurface of the sampleusing lock in detection.
 2. The method of claim 1, wherein the method isused by a system that is configured for near field holography.
 3. Themethod of claim 1, wherein the method is used by a system that isconfigured for near field holography for nano-mechanical imaging ofburied defects and structures.
 4. The method of claim 1, wherein themethod is used by a system that is configured to recover surfaceacoustic wave phase information of an interaction between thecantilevered tip and the surface, and wherein the recovered informationallows measurements of viscoelastic properties and enables applicationof acoustic holography algorithms for imaging nano-scale sizedsubsurface defects.
 5. The method of claim 1, wherein the extractingstep comprises extracting surface acoustic wave amplitude and phaseinformation associated with the surface and subsurface of the sampleusing lockin detection and a scanning near field ultrasound holographyelectronic module.
 6. The method of claim 1, wherein a scanning nearfield ultrasound holography electronic module extracts beats that serveas a reference for a lockin amplifier.
 7. The method of claim 1, whereinthe lockin amplifier receives beats extracted by an electronic module,the beats serving as a reference for the lockin amplifier, and whereinthe lockin amplifier receives a differential output of an opticaldetector, the optical detector being used to detect movement of acantilever with the cantilevered tip.
 8. The method of claim 1, whereina scanning near field ultrasound holography electronic module extractsproducts of fundamental and harmonic signals that serve as a referencefor a lockin amplifier.
 9. The method of claim 1, wherein the lockinamplifier receives products of fundamental and harmonic signalsextracted by an electronic module, the products serving as a referencefor the lockin amplifier, and wherein the lockin amplifier receives adifferential output of an optical detector, the optical detector beingused to detect movement of a cantilever with the cantilevered tip. 10.The method of claim 1, wherein the second ultrasonic frequency vibratesthe sample at a frequency offset from the first ultrasonic frequency.11. The method of claim 1, wherein the extracting step includesextracting subsurface mechanical data regarding the sample from the tipdeflection signal.
 12. The method of claim 1, wherein the subsurfacemechanical data includes interfacial bonding.
 13. The method of claim 1,comprising imaging a spatial variation of surface and subsurfaceviscoelastic phase.
 14. The method of claim 1, comprising defining acharacteristic viscoelastic response time of the sample based on theamplitude and phase information.
 15. The method of claim 1, comprisingmaintaining the vibration of a cantilever tip piezo at a tip piezoresonance frequency and the vibration of the sample at a sampleresonance frequency.
 16. The method of claim 15, comprising providingelectrical feedback to maintain the tip piezo resonance frequency andthe sample resonance frequency.
 17. The method of claim 1, wherein beatfrequencies, product frequencies or frequency addition are used togenerate the tip deflection signal to monitor the sample in near contactmode.
 18. The method of claim 1, comprising operating in near contactmode without contact between the cantilevered tip and the sample. 19.The method of claim 1, wherein the vibrating of the cantilevered tip isfacilitated by a first piezo crystal attached to a substrate of thecantilevered tip, wherein the vibrating of the sample is facilitated bya second piezo crystal attached to a base of the sample, and wherein thefirst piezo crystal and the second piezo crystal are matched.
 20. Themethod of claim 1, wherein the cantilevered tip and the sample arevibrated at fundamental resonance frequencies and related harmonics. 21.A scanning near field holography method for surface and subsurfaceimaging comprising: vibrating a cantilevered tip at a first microwavefrequency; vibrating a sample at a second microwave frequency, the firstmicrowave frequency being different from the second frequency; detectingmovement of the cantilevered tip interacting with the vibrating sampleto provide a tip deflection signal; and extracting surface acoustic waveamplitude and phase information associated with the subsurface of thesample from the tip deflection signal using lock in detection.
 22. Themethod of claim 21, wherein the method is used by a system that isconfigured for near field holography.
 23. The method of claim 21,wherein the method is used by a system that is configured for near fieldholography for nano-mechanical imaging of buried defects and structures.24. The method of claim 21, wherein the method is used by a system thatis configured to recover surface acoustic wave phase information of aninteraction between the cantilevered tip and the surface, and whereinthe recovered information allows measurements of viscoelastic propertiesand enables application of acoustic holography algorithms for imagingnano-scale sized subsurface defects.
 25. The method of claim 21, whereinthe extracting step comprises extracting SAW amplitude and phaseinformation associated with the subsurface of the sample from the tipdeflection signal using lock in detection and a scanning near fieldultrasound holography electronic module.
 26. The method of claim 21,comprising: using a linear interaction between the cantilevered tip andthe sample to detect beats at the surface of the sample; and extractingphase and amplitude signals for the first microwave frequency and thesecond microwave signal.
 27. The method of claim 21, comprising imagingthe surface and subsurface of the sample based on the amplitude andphase information of the tip deflection signal.
 28. The method of claim21, wherein the cantilevered tip and the sample are vibrated atfundamental resonance frequencies and related harmonics.
 29. An atomicforce microscopy system, comprising: a cantilever, wherein thecantilever includes a tip at an end of the cantilever; a deviceconfigured to supply vibrations at a first frequency to the cantileverto generate vibrations at the tip; and a detector configured to detectmovement of the tip based on an atomic force between the tip and asurface of a sample, wherein the sample is vibrated at a secondfrequency that is different from the first frequency, wherein the systemis configured for near field holography for nano-mechanical imaging ofburied defects or buried structures.
 30. The system of claim 29, whereinthe system is configured for near field holography.
 31. The system ofclaim 29, wherein the system is configured to perform nondestructiveimaging of the buried defects or the buried structures at ananometer-scale resolution when the tip is operated in near contactmode.
 32. The system of claim 29, wherein the system is configured torecover surface acoustic wave phase information of an interactionbetween the tip and the surface, and wherein the recovered informationallows measurements of viscoelastic properties and enables applicationof acoustic holography algorithms for imaging nano-scale sizedsubsurface defects.
 33. The method of claim 1, comprising: selecting, bya circuit, at least one of beat frequencies and product frequencies thatserve as a reference for use with the lock in detection, the circuitbeing communicatively disposed between at least one function generatorand a lockin amplifier.
 34. The method of claim 21, comprising:selecting, by a circuit, at least one of beat frequencies and productfrequencies that serve as a reference for use with the lock indetection, the circuit being communicatively disposed between at leastone function generator and a lockin amplifier.
 35. The system of claim29, comprising: a function generator configured to generate a signal ofat least one of the first microwave frequency and the second microwavefrequency; and a circuit configured to select at least one of beatfrequencies and product frequencies that serve as a reference for usewith the lock in detection, the circuit being communicatively disposedbetween the function generator and a lockin amplifier.
 36. The system ofclaim 29, wherein the tip and the sample are vibrated at fundamentalresonance frequencies and related harmonics.